Highly branched polymeric compositions are known to those skilled in the art and are useful in a variety of solid-liquid separation applications, particularly in flocculation of various dispersions of suspended solids, such as sewage sludge, and in the thickening of cellulosic paper pulp suspensions. Modern concerns about environmental pollution and the increasing cost of materials have made it highly desirable to produce flocculating agents which improve the dewatering efficiency of suspended wastes.
It has now been found that highly branched, water-soluble, high molecular weight polymeric microparticles, wherein said microparticles have an average unswollen diameter of less than about 0.1 micron, a solution viscosity of at least about 1.8 mPa.s, as defined below, and having a branching agent content of at least about 4 molar parts per million, based on the monomeric units percent in the polymer, satisfy these demands, cationic microparticles having a solubility quotient of greater than about 30 percent.
The prior art discloses several attempts to produce high molecular weight, branched, water-soluble polymers. Zweigle, U.S. Pat. No. 4,059,522 discloses the use of branching agents to yield a totally crosslinked system, but flocculants prepared in this manner are insoluble in water and thus ineffective. Morgan, et al., U.S. Pat. No. 3,698,037 discloses branched, cationic homopolymers obtained through incorporation of a multifunctional branching agent in the absence of a molecular weight controlling or chain-transfer agent. Said inventors allege that while insoluble products are formed by gel or suspension polymerization techniques, soluble products are obtained through emulsion polymerization despite the use of higher concentrations of crosslinking agents. However, said inventors failed to set forth any data showing the emulsion polymers to be truly soluble. It is a well known fact to those skilled in the art, that it is very difficult to discern the true solubility characteristics of oil-in-water emulsion polymers since the dispersed oil gives rise to an opaque dispersion which masks the visual appearance of insoluble material which may be present. Solubility characteristics are further masked since emulsion polymerization produces polymer particles in the range of from 1 to 10 microns in diameter which particles are too small to be detected by the unassisted eye. It is also well known to those skilled in the art, that the molecular weight of such cationic homopolymers is limited, whereas much higher molecular weights may be obtained by copolymerizing with non-ionic monomers. Furthermore, conventional inverse emulsion polymerization rather consistently produces particles with average diameter ranging from about 1 to about 10 microns.
Pech, Fr. 2,589,145 discloses a branched copolymer prepared using solution polymerization techniques in the presence of a high activity transfer agent. However, the polymers disclosed therein comprise molecules having molecular weights below 1 million with solution viscosities in the range of from 2200 to 3600 mPa.s at 20 percent polymer concentrations, thus showing these polymers to be truly low molecular weight polymers. Furthermore, polymeric microparticles having an average diameter of less than 0.1 micron cannot be prepared through the Pech solution polymerization teaching, even in the presence of a chain-transfer agent.
Other patent disclosures, Whittaker, U.S. Pat. No. 4,705,640; Flesher, et al., U.S. Pat. No. 4,720,346; and Farrar, U.S. Pat. No. 4,759,856; teach shearing of the crosslinked polymer chains to obtain desired water-solubility. However, shearing requires expensive mechanical equipment and is inconvenient for the end-user. Furthermore, shearing is ineffective to produce the microparticles of this invention having an average diameter of less than 0.1 micron.
Water-insoluble, highly crosslinked, acrylamide latices or microgels are disclosed in Leong, et al., Inverse Microemulsion Polymerization, J. of Phys. Chem., Vol. 86, No. 13, Jun. 24, 1982, pp. 2271-3, wherein the author uses a 100:1 mixture of acrylamide:methylenebisacrylamide. No use of cationic or anionic monomers is mentioned.
Unexpectedly, water-soluble, highly branched, high molecular weight, cationic, non-ionic and anionic, polymeric microparticles are prepared by the practice of the processes of the present invention. These microparticles, when dissolved in water, surprisingly are excellent flocculating agents, producing highly efficient dewatering and thus satisfy a long felt need in the art.